1. Field of the Invention
The invention concerns the production of paper tablets by adhesively edge-padding a stack of paper with a latex adhesive and, after allowing the adhesive to dry, cutting the stack into individual tablets. The term "tablet" is here used to encompass books, reports, ledgers, and the like.
2. Description of the Related Art
Paper tablets can be assembled by brushing an adhesive onto one edge of a stack of paper. Usually, the stack includes a piece of chipboard (cardboard) that forms the base of each tablet. After the adhesive has dried, the stack is cut by hand with a knife at the underside of each piece of chipboard to produce individual tablets, each of which has an adhesive edge-padding.
In use, individual sheets of the tablet can be successively peeled from the adhesive edge-padding and sometimes are torn in doing so. The removal of a number of sheets typically leaves an upstanding ridge of the adhesive edge-padding which gradually becomes unsightly due to fragments of torn sheets and also due to pieces of the upstanding ridge breaking off from time to time. When a piece of the ridge is torn off, it sometimes carries with it part of the edge-padding for the underlying sheets, thus exacerbating the unsightliness of the tablet and sometimes causing the remaining sheets to become loose.
Three of the leading adhesives for making adhesively edge-padded tablets are "Merit" padding adhesive from Merrit Products Co., Painesville, Ohio; "Hurst Graphics" padding compound from Hurst Graphics, Los Angeles, Calif.; and "Champadco" padding cement from Champion Mfg. Co., Charlotte, N.C. Each of these is believed to be a latex based upon polyvinyl acetate containing a plasticizer such as an ortho-phthalate ester and a viscosity-modifying agent. While claims have been made that such edge-padding adhesives dry within about one-half hour, it has been found that when used to form a stack of paper tablets, such edge-paddings require overnight drying before the stack is cut into individual tablets. Any effort to cut the stack sooner tends to cause the adhesive edge-padding to gum up, making it impossible to keep the knife from veering off. Such drying time virtually precludes large-scale manufacture, because of the large space that would be required for allowing the stacks to dry.
Because of the foregoing problems, the edges of many paper tablets are bound like paperback books, and the sheets are perforated to permit individual removal. Such tablets are not only more expensive to assemble, but there is waste in the portions of the paper sheets that remain with the binding.
3. Other Prior Art
A large number of latex adhesives are based on ethylene/vinyl acetate (EVA) copolymer which is inexpensive and can be compounded with inexpensive materials to form strong bonds to a great variety of substrates. For a discussion of EVA copolymers, see a brochure entitled "Airflex 300 Base for Adhesives" from Air Products and Chemicals, Inc., 1980. That brochure says that "Airflex" 300 is an EVA copolymer and lists 22 uses including "Bookbinding" and "Padding". It teaches that the viscosity of an EVA latex can be increased with additives such as cellulosic thickeners and plasticizers such as dibutyl phthalate; that plasticizers also improve specific adhesion; that wet tack can be enhanced by adding tackifying resins; and that the setting speed of the latex "can be improved by adding plasticizer, solvents or tackifying resins."
For, a discussion of styrene/butadiene (SB) copolymers, see a brochure entitled "Dow Designed Latexes For speciality Applications," Form No. 191-193-86 from Dow Chemical Co.
We are not aware of any latex adhesive that is based on either EVA or SB copolymer and is specifically formulated and marketed for adhesively edge-padding paper tablets.